An aqueous hydrogen peroxide solution is widely used in many fields, for example, for bleaching paper and pulp and as a component in chemical polishing fluids. In recent years, the aqueous hydrogen peroxide solution has increasingly been used in the electronic industry, for example, as a cleaning agent for silicon wafers and as a cleaning agent in production processes of semiconductors. Accordingly, there is a demand for an aqueous hydrogen peroxide solution of enhanced quality in purity as obtained by minimizing the content of various impurities in the aqueous hydrogen peroxide solution.
Generally, hydrogen peroxide is now produced exclusively by the anthraquinone process. In the anthraquinone process, first, a derivative of anthraquinone, such as a 2-alkylanthraquinone, is hydrogenated into anthrahydroquinone in the presence of a hydrogenation catalyst in a water-insoluble solvent. Subsequently, the catalyst is removed, and the reaction product is oxidized with air. Thus, not only is the original 2-alkylanthraquinone regenerated but also hydrogen peroxide is produced at the same time. The produced hydrogen peroxide is extracted from the oxidation product with water to thereby obtain an aqueous solution containing hydrogen peroxide. This process is generally known as the anthraquinone autoxidation process. The aqueous hydrogen peroxide solution produced by the anthraquinone autoxidation process contains inorganic ion/compound impurities, such as Al, Fe, Cr, Na and Si, attributed to, for example, the materials constituting the apparatus. Therefore, the aqueous hydrogen peroxide solution is subjected to purification operation for removing such impurities to thereby attain a high purity in accordance with the required quality in particular use.
Especially in the electronic industry, an extremely high purity is required for the aqueous hydrogen peroxide solution. It is required that, in the aqueous hydrogen peroxide solution, the content of organic impurities be not greater than 10 ppm and the content of metal ion impurities be not greater than 1 ppb. For the removal of impurities from the aqueous hydrogen peroxide solution, it is generally known to employ an ion exchange resin, a chelate resin, an adsorption resin or the like. When the removal of impurities is carried out on an industrial scale with the use of such a resin, there is commonly employed the continuous liquid pass method (tower process) which ensures high operation efficiency and high removing efficiency.
The purification of aqueous hydrogen peroxide solution by the tower process involves such a problem that bubbles are formed by autolysis of hydrogen peroxide, which is a property peculiar to hydrogen peroxide, and the bubbles stick to resin circumstances to thereby lower purification efficiency, i.e., impurity removing efficiency.
As a means for solving this problem, for example, Japanese Patent Laid-open Publication No. 9(1997)-77504 discloses a process in-which an upper part of an ion exchange resin tower is pressurized so as to increase the solubility of bubbles formed by autolysis of hydrogen peroxide, thereby eliminating bubbles from the purifier tower.
However, the process disclosed in Japanese Patent Laid-open Publication No. 9(1997)-77504 has a drawback in that the content of metal ion impurities in purified aqueous hydrogen peroxide solution is 1 ppb, which is not necessarily satisfactory level, and that quality reproducibility is poor. Moreover, when the operation time is prolonged, it may occur that bubbles are accumulated in the ion exchange resin tower with the result that the area of contact between ion exchange resin and aqueous hydrogen peroxide solution is decreased, or the complete adsorption band (part where the adsorption of impurity ions has been completed) or exchange band (part where ion exchange is being performed) of ion exchange resin is disordered. Consequently, satisfactory removal of impurities may be inhibited, and further the passing of aqueous hydrogen peroxide solution may be hindered to thereby bring about problems such as pressurization and temperature rise within the ion exchange resin tower.
In these circumstances, the inventors have made extensive and intensive studies with a view toward solving the above problems. As a result, it has been found that, when the aqueous hydrogen peroxide solution is purified by controlling the output of a feed pump for charged aqueous hydrogen peroxide solution in cooperation with a flow sensor capable of sensing a flow rate of charged aqueous hydrogen peroxide solution being fed to a purifier tower so as to bring the charged aqueous hydrogen peroxide solution into contact with an ion exchange resin while maintaining the flow of charged aqueous hydrogen peroxide solution at a constant rate, the impurities of aqueous hydrogen peroxide solution can be removed to the order of ppt (parts per 1012). It has also been found that, in this purification process, not only is the reproducibility of impurity removing level very high but also the pressurization and temperature rise during purification can be avoided to thereby realize a safe purification of aqueous hydrogen peroxide solution. The present invention has been completed on the basis of these findings.
When the aqueous hydrogen peroxide solution is purified by bringing the aqueous hydrogen peroxide solution into contact with an ion exchange resin, a chelate resin or an adsorption resin while maintaining the flow of aqueous hydrogen peroxide solution being fed to the purifier tower at a constant rate according to the present invention, sticking of bubbles to the ion exchange resin, etc. within the purifier tower can be suppressed. Further, in this purification process, not only can the leaving of bubbles in the purifier tower be avoided but also disordering of the complete adsorption band or ion exchange band can be suppressed. Still further, in this purification process, the aqueous hydrogen peroxide solution can be easily passed through the purifier tower. Thus, a purification efficiency of aqueous hydrogen peroxide solution is high.
It is an object of the present invention to provide an apparatus for producing a purified aqueous hydrogen peroxide solution, which process is free from any disordering of ion exchange band during purification, free from bubbles remaining in a purifier tower and free from any pressurization or temperature rise within the purifier tower to thereby enable effecting a safe and efficient contract of aqueous hydrogen peroxide solution with an ion exchange resin or the like.